There is a shell-type sheet including a seating portion configured to support an occupant in a seating posture and a shell portion configured to cover the circumference of the seating portion from a rear of the occupant (see JP-A-2015-020527). In such a seat, the shell portion is fixedly disposed to be tilted rearward, and the seating portion is movable in response to the occupant's preference in space in front of the shell portion. In this structure, however, since the shell portion is fixed at the rear even when the seating portion is used in a front position, a wasted space can be formed between the seating portion and the shell portion. For this problem, it is considered that the shell portion is also movable depending on the position of the seating portion.
In this case, however, when the occupant sets a normal mode in order to take a general seating posture in which a reclining angle of the seating portion and the shell portion is relatively small, a height from the installation floor surface of the seat becomes higher, resulting in causing a problem that a gap between an upper end of the shell portion and a ceiling of a vehicle becomes smaller. As a result, all-round visibility of the occupant becomes narrower, and thus an tightness feeling is imparted to the occupant.